Re: stack usage in toast_insert_or_update()

On 1/31/2007 12:41 PM, Tom Lane wrote:
> "Pavan Deolasee" <pavan(dot)deolasee(at)gmail(dot)com> writes:
>> On 1/31/07, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> The toast code takes pains to ensure that the tuples it creates won't be
>>> subject to re-toasting. Else it'd be an infinite recursion.
>
>> I think I found it. The toast_insert_or_update() function gets into an
>> unnecessary recursion because of alignment issues. It thus toasts
>> already toasted data. This IMHO might be causing unnecessary
>> overheads for each toast operation.
>
> Interesting --- I'd never seen this because both of my usual development
> machines have MAXALIGN 8, and it works out that that makes
> TOAST_MAX_CHUNK_SIZE 1986, which makes the actual toasted tuple size
> 2030, which maxaligns to 2032, which is still less than
> TOAST_TUPLE_THRESHOLD. I think the coding was implicitly assuming that
> TOAST_TUPLE_THRESHOLD would itself be a maxalign'd value, but it's not
> necessarily (and in fact not, with the current page header size ---
> I wonder whether the bug was originally masked because the page header
> size was different??)
>
> We can't change TOAST_MAX_CHUNK_SIZE without forcing an initdb, but I
> think that it would be safe to remove the MAXALIGN'ing of the tuple
> size in the tests in heapam.c, that is
>
> if (HeapTupleHasExternal(tup) ||
> (MAXALIGN(tup->t_len) > TOAST_TUPLE_THRESHOLD))
> heaptup = toast_insert_or_update(relation, tup, NULL);
> else
> heaptup = tup;
>
> becomes
>
> if (HeapTupleHasExternal(tup) ||
> (tup->t_len > TOAST_TUPLE_THRESHOLD))
> heaptup = toast_insert_or_update(relation, tup, NULL);
> else
> heaptup = tup;
>
> which'll save a cycle or two as well as avoid this corner case.
> It seems like a number of the uses of MAXALIGN in tuptoaster.c
> are useless/bogus as well. Comments?

Can't we maxalign the page header in the calculations?

Jan

>
> regards, tom lane
>
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